1. Field of the Invention
This invention relates to a method and an apparatus for crushing the crusts of seeds, trunks, branches, leaves, etc. of plants belonging to a family of grasses. More particularly, it is a compressing and grinding apparatus which applies pressure and heat to the crusts of seeds of cereals having a strong cell structure, such as rice hulls, and crushes them until even their cells are destroyed.
2. Description of the Prior Art
The crusts of seeds of any plant belonging to a familty of grasses, such as rice hulls, and the trunks, branches and leaves of plants of rice, barley or wheat, millet, etc. are very difficult to crush, as they contain a siliceous substance. Therefore, they can hardly be absorbed into the body of an animal or a plant. Even if they can be crushed into fine particles, they cannot be used as a filler or modifier for any synthetic high molecular compound, as they have only a low degree of compatibility with the compound, or a low degree of dispersibility therein. The disposal of a large amount of rice hulls, etc. resulting from threshing presents a big problem.
There is known an apparatus for crushing any such material. It comprises a screw and an outer cylinder which defines therebetween a space having a gradually decreasing cross-sectional area. The material to be crushed is fed through the space and is thereby ground and kneaded. There is also known a ball mill which can grind various kinds of materials into fine particles. There is, however, not known any apparatus that is suitable for grinding into fine particles the crusts of cereal seeds, such as rice hulls, having a strong cell structure and a volume which is greatly reduced when they are crushed. There has long been a strong demand for an apparatus which is effective for such purposes.
I, the inventor of this invention, thought that it would be effective to employ a method comprising feeding rice hulls, etc. into a space defined between a screw and an outer cylinder and having a gradually decreasing cross-sectional area, and grinding them under pressure and heat. I have, therefore, been making extensive efforts to obtain an improved apparatus which can effectively carry out the method. A maximum normal stress of 6 to 7 tons/cm.sup.2 and a maximum shearing stress of 2 tons/cm.sup.2 bear on the thread at the leading end of the screw. It is heated to a high temperature in the range of 450.degree. C. to 500.degree. C. by the heat of friction and the heat which is supplied from an external source. Therefore, the screw in the conventionally available apparatus got worn so easily at the end thereof that it could withstand use for only several hours. It was necessary to change the screw very often. In order to obtain a screw of improved wear and impact resistance, I have tried to harden the surface of the screw, which is formed from tool steel, by coating it with a ceramic material. The surface of the screw is, however, heated to a high temperature by the heat of friction and the heat applied thereto, as hereinabove stated. The difference in thermal expansibility between the steel and the ceramic material causes the development of a large internal stress in the screw. This stress and the external stress as hereinabove stated give rise to the separation of the ceramic coating from some portions of the screw and thereby the exposure of the steel in those portions. The wear of those portions proceeds to a greatly increased degree and eventually results in the wear of the screw as a whole. The screw can withstand use for only several days. Whenever the screw becomes unusable, it is necessary to discontinue the use of the apparatus and disassemble it to change the screw.
The serious drawbacks of the conventional apparatus as hereinabove pointed out are due to the construction and material of the screw, as well as of the outer cylinder. The apparatus has only a low degree of operating efficiency or productivity. The use of a large number of screws disables a reduction in the cost of the grinding operation.